In a typical electrostatic image reproduction machine, a photoconductive member is electrostatically charged and radiated with a light image corresponding to original information to form an electrostatic latent image on the member through localized photoconduction. A development station applies toner (pigmented marking particles) to the member to create a toned image which is transferred and fixed to a receiver sheet to provide a permanent reproduction. A cleaning station then cleans off any residual toner substance from the member in preparation for another copying operation.
Commonly assigned, copending U.S. patent application Ser. No. 621,691, filed on Nov. 30, 1990, in the names of DeBoer et al, now abandoned, discloses another type of image reproduction machine in which a uniform layer of toner particles is applied to a receiver sheet having a thermoplastic outer layer. The receiver sheet is then imagewise exposed by a laser to soften the thermoplastic layer in exposed areas. Toner particles adjacent the softened areas migrate into the thermoplastic layer under the influence of an electrostatic attraction of the particles to the receiver sheet. After laser exposure is complete, the thermoplastic layer cools, adhering the migrated toner particles to the layer. The toner particles which did not migrate into the thermoplastic layer are then removed by a magnetic brush cleaner. This leaves a toned image in/on the receiver sheet. The image is fused with heat to permanently fix it to the receiver sheet. In this process a large quantity of toner particles must be removed from the receiver sheet to create the toner image.
As far back as 1959, it was asserted by Greig, in U.S. Pat. No. 2,874,063, that developer mix, comprised of toner and magnetic carrier particles, used to develop an electrostatic latent image could, if substantially depleted of toner particles, be used to clean a photoconductive surface of residual toner. This suggests that one apparatus could be used for both development and cleaning.
In U.S. Pat. No. 4,142,165, issued Feb. 27, 1979, in the names of Miyakawa et al, a magnetic brush is disclosed which both develops electrostatic latent images with toner and cleans residual toner. A magnetic brush, comprised of a plurality of magnets fixedly mounted about an axis and a nonmagnetic cylindrical sleeve surrounding the magnets, has its lower portion immersed in developer mix in a developing tank. The developer mix is comprised of magnetic carrier particles and toner particles which are attracted to and adhere to each other due to triboelectric charging. The sleeve rotates about the magnets and, due to the magnetic attraction of the carrier particles to the magnets, developer mix is attracted to and adheres to the periphery of the sleeve to form a rotating magnetic brush. A doctor blade integral with the development tank limits the thickness of the magnetic brush. During development, the rotating magnetic brush is brought into contact with a photoconductive surface. Toner particles of a charge opposite that of an electrostatic latent image on the photoconductive surface are electrostatically attracted from the magnetic brush to the electrostatic image to form a toned image on the photoconductive surface. This toned image is transferred to a receiver sheet to which it is permanently fixed. During cleaning, a bias voltage is established between the magnetic brush and an electrode roller. This causes toner particles to be transferred from the magnetic brush to the electrode roller. The magnetic brush, then substantially depleted of toner particles, is able to clean residual toner particles from the photoconductive surface. Toner particles on the surface of the electrode roller are scraped off by a skive into a container.
A major problem with the Miyakawa et al apparatus is that during the cleaning step the nonmagnetic sleeve remains immersed in the developer mix and continues to pick up fresh developer mix. The electrode roller thus ends up "cleaning" a lot of toner from the magnetic brush that did not come from the photoconductive surface but rather, came from the fresh developer mix. This method of cleaning is inefficient and would not work well in a copier, such as the one disclosed in the DeBoer et al application, wherein a large amount of toner particles (not just residual toner) must be removed from a surface. A further problem with this apparatus is that toner particles removed from the electrode roller cannot be returned directly to the developing tank.